Fibre-based capacity model for URM piers subjected to combined in-plane and out-of-plane actions

Seismic performance of masonry buildings is usually assessed at local and global levels, separately. Seismic safety against local collapse mechanisms is evaluated via macroblock models and linear/nonlinear kinematic analysis methods. Conversely, safety against in-plane failure modes is assessed through nonlinear static/dynamic analysis of global macroelement models. Nonetheless, the in-plane seismic capacity of masonry walls can be strongly influenced by simultaneous response to out-of-plane actions. Although this issue has been recently investigated in a few experimental programs and numerical studies, the level of knowledge is still limited. In this study, the authors present a novel fiber-based capacity model that allows performance-based seismic design/assessment of unreinforced masonry piers subjected to combined in-plane and out-of-plane loading. Based on a nonlinear incremental analysis procedure, moment–curvature diagrams are derived at different levels of axial load and 3D flexural strength domains are developed at five performance limit states. Nonlinear sectional capacity under biaxial bending and axial loading is directly governed by the macroscopic constitutive model assigned to masonry and sectional shape. Analysis results show a strong interaction between bending moments related to in-plane and out-of-plane loading, which changes with the axial load level. Simplified biaxial interaction models are derived through nonlinear regression analysis for engineering practice. It is shown that the axial load level and ratio between in-plane and out-of-plane actions has an impact on sectional ductility at different limit states. The capacity model allows considering the softened response of masonry sections under increasing axial load levels, which also induces a reduction in ultimate axial load.